It is known to provide security enclosures with walls or sheets incorporating coils, meshes or grids of electrically responsive material and to monitor certain electrical characteristics of the material to provide an indication when the sheet containing the material has been pierced or an attempt has been made to open the enclosure through other means. Detection of such an attempt may activate an alarm, destroy the material or object within the enclosure, or erase information if the object within the enclosure, for example, contains sensitive magnetically recorded information. Examples of such enclosures are disclosed in PCT International Application Publication No. WO87/06749 to Wolf, U.K. Patent No. 1,375,926 to GAO Gesellschaft Fur Automation und Organisation mbH, U.S. Pat. No. 4,785,743 to Dalphin and European Patent Publication No. 277,679 to Seculock BV. However, the present invention is more closely related to security enclosures of the form disclosed in U.K. Patent Application No. GB 2220513A, to W. L. Gore and Associates, Inc. The disclosed security enclosures are formed from layers of flexible material including a matrix of diagonally extending semi-conductive lines printed onto a rectangular thin insulating film. The matrix of lines forms a continuous conductor which is broken if attempts are made to penetrate the film. The circuit is monitored by opening the conductor at one point and measuring the change of resistance between the two ends of the circuit.
The Gore security enclosure also includes a further protective laminate comprising two flexible layers of semi-conductive fibrous material separated by a layer of insulating material. The lengths of the fibres of the fibrous material are greater than the thickness of the layer of insulating material so that piercing the enclosure forces fibres from one fibrous layer to penetrate the insulating layer and contact the other fibrous layer to produce a detectable change in an electrical characteristic of the layers.
Further, the Gore application discloses enclosures in the form of shallow, rectangular envelopes formed simply by folding the rectangular laminate about a single axis and then securing the edges of the laminate to one another to form the envelope.
Security enclosures in the form of wedge-shaped, cuboid and cube form are disclosed in U.K. Patent Application GB 2258075A (W. L. Gore & Associates (UK) Ltd). in which a laminate is folded about a plurality of fold lines to form the enclosures. This facilitates formation of enclosures for containing objects that are other than thin and flat.
Further improved security enclosures are also described in the W. L. Gore & Associates (UK) Ltd. U.K. Patent Applications GB 2256956A, GB 2256957A and GB 2256958A, each of which describes features for increasing the difficulty of gaining unauthorised access to the enclosure. In GB 2256956A two layers of semi-conductive fibrous material are separated by a layer of auto-pyrotechnic insulating material. In addition to detecting physical penetration, as discussed above with reference to GB 2220513A, if a laser or other elevated temperature cutting means penetrates the layers the laser ignites the auto-pyrotechnic material and the subsequent decomposition of the layer will result in the fibrous layers coming into contact with one another.
In GB 2256957A, a matrix of semi-conductive lines is printed on a thin insulating film, the matrix of lines forming a plurality of conductors which are individually monitored. The lines extend over both sides of the insulating film and are joined at the edges of the film to define the conductors, each of which comprises a large number of line segments. The configuration of the connections between the lines may be varied such that the conductors may be formed of different configurations of lines. Thus, it would be difficult for an intruder to predict the combination of line segments which defined a particular conductor. In the preferred embodiment each conductor is of the same electrical length such that the configuration of the connections between the conductors and a monitoring circuit may also be varied, further increasing the difficulty of gaining access to the enclosure by isolating individual conductors from the monitoring circuit.
In GB 2256958A a low tensile strength layer is provided between a monitored conductor and a first insulating film, and a second insulating film is adhered over the conductor. An attempt to remove the second insulating film to gain access to the conductor results in break-up of the low tensile strength layer and detectable damage of the conductor.
The preferred embodiment described in GB 2256958A also includes semi-conductive fibrous layers separated by an insulating layer as described in GB 2220513A. However, in addition, two layers of semi-conductive low melt material are provided, one on each side of the insulating layer. If an attempt is made to penetrate the laminate using a laser the low melt material will melt and flow through any breaks in the insulating layer to form a detectable electrical connection between the semi-conductive fibrous layers. A similar arrangement is also described in European Patent Application No. 0459838.
When forming an enclosure from such laminates it is of course necessary to overlap edges of the laminates to define the enclosure. To ensure the security of the enclosure the overlapping edges must be carefully formed such that separation of the edges, held together by an adhesive, does not permit unauthorised entry to the enclosure. An example of an overlapping edge configuration is disclosed in GB 2256958A (FIG. 3) in which the layers of the laminate are feathered and an additional layer of low tensile strength material is provided below conductive lines provided at one of the edges. While providing a secure join, the necessity to feather the edges of the various layers of the laminate and provide additional layers of material at the edges of the laminate adds to the complexity and expense of forming the enclosure.
Further, when forming box-like security enclosures, such as described in GB 2258075A, it is desirable to fold the laminate on itself to allow the creation of corners between walls of the enclosure. It would of course be possible to form the laminates such that such folding was not required, for example by utilising a cruciform-shaped laminate to form a cuboid enclosure, though this would increase the difficulty of forming the layers on the laminate, particularly the lines which form the conductors, due to the more complex shapes which would be required, and would produce less secure areas in the enclosure between the edges of the laminate which would be brought together to define the corners of the enclosure. Thus, folding the laminate on itself is considered a desirable step, though there are still potential weak spots at such folds, for example where electrically insulating layers are brought together, as the layers could possibly be separated without damaging the detecting layers which lie below the insulation. Where two electrically responsive layers of lines would otherwise be brought together by folding, one half of each portion may be provided without such lines as proposed in GB 2258075A, though this increases the difficulty of forming conductors formed of such lines due to the more complex shapes which are required.
It is among the objects of the present invention to provide a method of forming a security enclosure defined by a laminate with overlapping edges in which such edges are secure and may be formed relatively easily.